The invention relates to a method for regulating the working point of a modulator. The modulator generates a modulated output radiation, for example in the visual range, from an input radiation as a function of a control signal.
Stable pulse sources are required to generate pulses in optical telecommunication transmission networks. A simple and cost-effective method for generating pulses from what is referred to as a continuous-wave source using high-speed optical modulators is described in German patent document no 199 24 347.6. However, the long-term stability of the pulse source is a problem with this method. In order to avoid shifting of the working point, stable modulators, in which long-term stability is achieved by means of costly structural measures, have been used at low data rates. The same problems occur with data modulators.
The present invention pertains to a simple method for regulating the working point of a modulator which ensures a stable working point of the modulator. In addition, the present invention pertains to an associated drive unit.
The invention is based on the fact that the working point is an essential operating parameter of the modulator. If the working point changes, the pulses generated by the modulator also change. The working point can be set very precisely when a modulator is manufactured, but it then drifts as a function of various causes. Such causes are, for example, aging of the modulator over the years or an operating temperature which changes within minutes while the modulator is operating, for example directly after switching-on.
The invention is also based on the fact that the working point can be easily set with respect to the transmission characteristic curve of the modulator by means of the average value of the control signal or using an auxiliary signal which ultimately influences the average value of the control signal. Furthermore, the invention is based on the idea that a deviation of the actual working point from a predefined setpoint working point results in a change in the output radiation.
In a method according to the invention, the average radiant power is sensed from the output radiation in at least one predefined frequency range. The average radiant power is the radiant power averaged over the frequencies. Furthermore, a periodic deflection of the working point in accordance with a working point deflection frequency is forcibly brought about. A regulating signal is generated as a function of the deflection of the working point. The average value of the control signal and/or the signal value of the auxiliary signal are changed as a function of the regulating signal in such a way that the deviation between the actual working point and the setpoint working point becomes smaller.
As a result of this procedure, both short-term deviations of the actual working point from the setpoint working point and long-term deviations due to a change in the transmission characteristic curve of the modulator can easily be compensated for. The average radiant power is used as a regulated variable. The voltage or the current of the control signal is used as the manipulated variable.
As a result of the reference to a prominent point, the regulation can also be carried out without predefining a setpoint power. For example, a minimum value, a maximum value, an inflection or another point at which a derivative has the value zero can be selected in the power curve as the reference point.
The methods known from regulating technology are used as regulating methods, for example, a proportional, a proportional-integral or a proportional-integral-differential regulating method. The power sensed can, if appropriate, be used directly as a regulated variable. However, very good control circuits are obtained if the regulated variable is sensed using phase-sensitive detection, which is also known as a lock-in method. Phase-sensitive detection has the advantage that the regulation can be carried out comparatively independently of interference variables, for example of signal noise. Phase-sensitive detection is explained, for example, in the book xe2x80x9cElectronic Measurement and Instrumentationxe2x80x9d, Klaas B. Klaassen, Cambridge University Press, 1996, pages 204 to 210.
In one embodiment, a derivative of the function of the working point and sensed power is used as the regulated variable. During the regulation operation, it is then possible to make reference to a point of the function at which the selected derivative has the value zero. Making reference here means that regulation is performed to the regulating point without detuning the control circuit.
The modulator is either a pulse modulator which is driven with a periodic control signal with a predetermined driving frequency, or a data modulator which is driven with a control signal which is dependent on the data to be transmitted, half the data rate being referred to as the driving frequency.
In an embodiment, the predefined frequency range contains all the frequencies of the frequencies of the output radiation which can be sensed by a transducer unit. For example a photodiode or a phototransistor is used as the transducer unit. The frequencies which can be sensed by the transducer unit are determined by its design. In addition to the transducer unit, no filters for filtering out specific frequency ranges are necessary in this embodiment. The predefined frequency range can have a very broad band, for example from 0 Hz to the gigahertz range. However, it is also possible to use transistor units which operate with a comparatively narrow band, sensing, for example, only frequencies from 0 Hz to the kilohertz range. Narrow-band transducer units can be manufactured more easily in comparison to broadband transducer units and can therefore be obtained more cost-effectively.
In another embodiment, the predefined frequency range contains only a portion of the frequencies of the output radiation which can be sensed by a transducer unit. This portion is determined by the design of a filter unit connected downstream of the transducer unit. The filter unit is, for example, a low-pass filter, a bandpass filter or a high-pass filter. In this development, changes which occur in the spectrum as a function of the working point are used. As a result of the selection of one or more suitable frequency ranges it is possible to obtain very large signal differences between the power in the setpoint working point and the power when there are deviations from the setpoint working point.
In one refinement of the method with a filter unit, the predefined frequency range includes a frequency which corresponds to the driving frequency. Twice the driving frequency and multiples of twice the driving frequency are not contained in the frequency range. The refinement is based on the fact that when there are deviations from the setpoint working point a considerable power increase occurs in the vicinity of the driving frequency in the power density spectrum. The power which can be sensed in the vicinity of the driving frequency is dependent on the magnitude of the deviation between the setpoint working point and actual working point.
If, in a further refinement of the method with a filter unit, in particular with a pulse modulator, the setpoint working point lies at a transmission maximum valuexe2x80x94what is referred to as RZ (return to zero) modexe2x80x94or at a transmission minimum valuexe2x80x94what is referred to as carrier suppressed RZ modexe2x80x94the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit, which is adjusted, without detuning, to a regulating point at which the average power within the predefined frequency range is at a minimum.
In another embodiment, the predefined frequency range contains only frequencies which lie far below the driving frequency, i.e. are low frequency in comparison to the driving frequency. For example, the frequencies are smaller than a tenth of the driving frequency. The signals to be processed thus have lower frequencies. Components are used which are configured for limiting frequencies which lie far below the driving frequency. If the driving frequency lies, for example, in the gigahertz range, components for the kilohertz range are suitable for processing because these components still sense the average power required for the regulation. Circuits required for the method can therefore be constructed cost-effectively without high-frequency components.
If the setpoint working point is at a transmission minimum value in a refinement with a low frequency rangexe2x80x94in particular in the case of a pulse modulatorxe2x80x94the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit which is adjusted to a regulating point at which the average power within the predefined frequency range is at a maximum.
If, on the other hand, the setpoint working point is at a transmission maximum value in an alternative refinement with a low frequency rangexe2x80x94in particular in the case of a pulse modulatorxe2x80x94the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit which is adjusted to a regulating point at which the average power within the predefined frequency range is at a minimum.
If the setpoint working point lies between a transmission maximum value and a transmission minimum value of the transmission characteristic curve (what is referred to as clock RZ mode) in a further alternative refinement with a low frequency rangexe2x80x94in particular in the case of a pulse modulatorxe2x80x94a regulating point at which the average power is at a minimum value or at a maximum value is selected.
If the setpoint working point lies between a transmission maximum value and a transmission minimum value, preferably at an inflection, in a further alternative refinement with a low frequency range in the case of a data modulator, the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit which is adjusted to a regulating point at which the function of the average power and of the working point has an inflection.
In another embodiment, the control circuit for regulating the working point is not detuned, so that the control circuit is regulated to the regulating point at the setpoint working point of the modulator. In what is referred to as the clock RZ mode, the control circuit is detuned.
A regulated variable with a correct sign can be easily acquired in an embodiment if a small deviation of the working point is forcibly brought about for regulating purposes. The power is then sensed at least two different working points. Phase-sensitive detection, for example, is based on forcibly bringing about such small deviations of the working point in such a way, said detection also being referred to as a lock-in method, see, for example, Klaas B. Klaassen, xe2x80x9cElectronic Measurement and Instrumentationxe2x80x9d, Cambridge University Press, 1996, pages 204 to 210.
In one refinement, for regulating purposes the deviation of the working point is forcibly brought about using a periodic deflection signal with a predefined deflection frequency. The deflection signal is preferably added to the control signal. A signal which is dependent on the sensed power is multiplied by a periodic reference signal whose frequency corresponds to the deflection frequency. A signal which results from the multiplication is used, after low-pass filtering and preferably after subsequent integration, to change the average value of the control signal and/or to change the signal value of the auxiliary signal. The limiting frequency of the low-pass filter determines the response time of the control circuit, which is, for example, between 10 milliseconds and 100 milliseconds. As a result of this method, the derivative of the power curve is ultimately used as a regulated variable. Depending on the phase of the deflection signal (xcfx80/2 or 3 xcfx80/2), the power can be regulated to a maximum value or a minimum value. The deflection signal has a cosine-shaped or sine-shaped profile. However, other deflection signals are also used, for example, signals with a square-wave profile. If the reference signal has a frequency which corresponds to a multiple of the deflection frequency, points can be detected at which higher derivatives are zero, for example an inflection at twice the deflection frequency.
At the same time as the working point, it is also possible to regulate the working range in a similar way. The deflection frequency for regulating the working point and the deflection frequency for regulating the working range are selected in such a way that the control circuits operate independently of one another. Thus, deflection frequencies which are different from one another are used, for example a deflection frequency of 3 kHz and a deflection frequency of 5 kHz.
The input radiation is generated in a pulse modulator or a data modulator using a continuous-wave light source or a pulsed radiation source. A pulse modulator forms a pulse light source, for example.
The driving frequency of the modulator is more than 1 gigahertz, preferably 5 gigahertz or 20 gigahertz, in some embodiments. In another embodiment, the modulator operates in the visual range. For example, the modulator contains a Mach-Zehnder interferometer. The transmission characteristic carve of the modulator is, for example, cosine-shaped or sine-shaped. However, modulators with other transmission characteristic curves are also used.
The invention also relates to a drive unit for carrying out the above mentioned methods. The technical effects which have been mentioned for the methods also apply to the drive unit and its embodiments.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures